I seem to have encountered a limitation of In Context modelling that I had not read about.
In the screen shot below, I planned to sweep a multibody rail part in the context of an assembly.
The limitation seems to be that I cannot select a sketch element which is not in the sketch I am editing.
This was not a problem when extruding normal to the sketch plane (as the graphic shows), but it prevents me relating an element in the rail profile sketch to an element in the track path sketch.
I know there are workarounds, but I want your confirmation that the limitation exists.